Roofing Insulation in between Rafters with Celotex

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Permalink Reply by dbh1954 on February 23, 2014 at 14:21

Hi, I've found this quite useful for looking up building regs.
http://www.planningportal.gov.uk/buildingregulations/

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Permalink Reply by 3n160pqkywsza on February 27, 2014 at 11:01

The U value requirement for areas of sloping ceiling is 0.18 W/m2K. The U value requirement for a flat ceiling with a loft void above is 0.16 W/m2K.

The exact solution will depend upon the size and centres of your rafters or joists and on the type of roofing membrane (breathable or traditional felt).

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Permalink Reply by Metso on February 28, 2014 at 12:26

Hi Jamie,

Thanks for your reply..

The rafters are 4x2"

with a gap of 11 1/2" in between each rafter.

what do you mean by centres of the rafters?

Thanks Adrian.

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Permalink Reply by 3n160pqkywsza on February 28, 2014 at 14:37

Hi Adrian,

The centres refers to the spacing’s between the rafters (which you have provided).

If you have a non breathable membrane (black roofing felt) then you could install 50mm of Celotex GA4000 between the rafters and 80mm of Celotex GA4000 below the rafters – maintaining a 50mm ventilated air path below the felt.

If you have a breathable membrane then you can install 75mm of Celotex GA4000 between the rafters and 55mm of Celotex GA4000 below the rafters.

Both solutions will achieve a U-value of 0.18 W/m2K.

Kind regards,

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Permalink Reply by Justin Guest on February 28, 2014 at 15:24

Jamie. I am following this thread as I may be doing a loft conversion soon. I am pretty keen to insulate the conversion as well as is physically possible. So, given what you have indicated above about the U value "requirement" (read minimum...), if this were your conversion, what would you be aiming to put in, knowing what you do about the subject?

Cheers

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Permalink Reply by Metso on February 28, 2014 at 17:44

HI Jamie,

Once again thank you for your reply.

I take it by your username, you either know alot about celotex or you're a fan... so my next question would be about buying the celotexGA4000, do you know any fantastically priced distributors?

Thanks Adrian

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Permalink Reply by 3n160pqkywsza on March 1, 2014 at 11:05

Hi Justin,

The standards required by the building regulations are updated every few years and are currently quite high. As long as you are meeting these requirements you can be assured that you will have a well insulated roof.

Of course there is nothing wrong with using more insulation and there is no upper limit on the amount you can use. Normally you would use the full depth of the rafter for insulation (minus any required ventilation space) and put as much insulation under the rafters as head room allows.

To offer an example, if you had rafters 150mm x 47mm at 400 centres with a traditional black felt you might do the following:

100mm of Celotex GA4000 between the rafters (leaving a 50mm ventilated air path below the felt)

50mm of Celotex GA4000 under the rafters.

This would achieve a U value of 0.17 W/m2K which is slightly better than the building regulations.

The insulation layer under the rafters can be increased to improve performance. However, as the thickness increases you need to consider the practicality of fixing the plasterboard through the insulation.

An good option for a really well insulated roof is to increase the thickness of insulation under the rafter and also introduce a counter batten between the insulation and the plasterboard.

For example (based upon the same roof build up):

100mm of Celotex GA4000 between the rafters (leaving a 50mm ventilated air path below the felt)

75mm of Celotex GA4000 under the rafters

25 x 47 battens (creating a 25mm airspace)

12.5mm plasterboard

The counter battens are fixed through the insulation to the rafters above and provide a framework of battens to screw the plasterboard to. This provides the following benefits:

Allows an easier fixing detail for the plasterboard

Provides a cavity to run services

The batten cavity provides additional thermal performance

The above example provides a U value of 0.13 W/m2K which offers a good level of performance over and above building regs.

Adrain, the best i can suggest for distributors are the list of stockists on the Celotex website. Here is the link: http://www.celotex.co.uk/find-a-stockist

Thanks,

Jamie

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Permalink Reply by Justin Guest on March 21, 2014 at 9:40

Jamie- I never thanked you for your response, much appreciated!

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Permalink Reply by 3t3jtnu2cvh11 on March 21, 2014 at 9:06

Thanks a ton for putting this question across. I have been breaking my head searching about the minimum permissible celotex thickness. Planning to get the insulation done at home soon, and these answers were of great help!

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